tests/installed: Fix TESTS= being empty
tests/installed: Fix TESTS= being empty

I broke this in https://github.com/ostreedev/ostree/pull/1509/commits/9b55aaea6f34b7094c44932a3c2e1cf2d54634fd
I'd obviously tested *setting* it locally worked, but I didn't test that
not having it set ran all the tests.

I don't understand why we were doing the `+ ` pattern before; let's
just check if it's empty.

Closes: #1516
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree